An astronaut landing on the Jovian moon Io would have a harsh environment to deal with, but would be rewarded with the most dazzling auroral light show in the solar system. A current study reveals new information about the moon's red, green and blue auroral lights and how they relate to Io's tenuous atmosphere.

Last October, a team of American and Taiwanese space scientists reported their discovery in images taken by the Galileo spacecraft of colorful auroral emissions from Io during eclipse by Jupiter. In tomorrow's issue (Aug. 6) of Science, they publish results from an in-depth study of those images.

The tenuous atmosphere of Jupiter's moon Io partially collapses in the darkness of the giant planet's shadow, they now find. At the same time, bright blue glows emanating from stealthy volcanic plumes grow even brighter.

"This is our first detailed look at visible aurorae on a solar system satellite", said Paul Geissler of the University of Arizona, lead author of the report. "The pictures help us to understand Io's atmosphere and the processes that generate the emissions."

Co-authors of the Science article are Alfred S. McEwen, also with the University of Arizona Lunar and Planetary Laboratory, Wing Ip of the Taiwan National Central University, Michael J. Belton of National Optical Astronomy Observatories, Torrence V. Johnson of the Jet Propulsion Laboratory in Pasadena, William H. Smyth of Atmospheric and Environmental Research in Cambridge, Mass., and Andy Ingersoll of the California Institute of Technology.

Io's aurorae, like those on Earth, are caused by the impact of electrons on atmospheric gasses. Io is bathed by a swarm of charged particles that are trapped by Jupiter's magnetic field, similar to the Van Allen radiation belts surrounding our own planet.

In addition, a powerful electric current flows from Io to the poles of Jupiter, caused by an enormous electrical potential some 400,000 volts generated by the motion of the jovian magnetic field past Io. When these electrons collide with the gasses in Io's atmosphere, they set off a dazzling light show of red, green and blue emissions bright enough to be visible to the naked eye. The red and green glows may be caused by neutral oxygen and sodium atoms, respectively, Geissler said. The bright blue emissions are probably due to sulfur dioxide vented from volcanoes on the moon's surface. Some of these plumes are invisible in daylight, owing to a lack of entrained dust particles, and can only be seen during eclipse, he added. The currents cause the gasses to light up, much the same as the glows from florescent lamps.

Io's eerie glow dims noticeably with time as the satellite lingers in Jupiter's shadow. The likely explanation, concludes the international team of scientists that analyzed the pictures, is a partial collapse of the moon's atmosphere during eclipse. Some of Io's patchy atmosphere is derived from sulfur dioxide ice on the surface of the satellite that is warmed by the Sun and sublimes (evaporates). This component probably begins to recondense in the absence of sunlight during eclipse. More surprisingly, the blue glows associated with volcanic plumes appear to intensify while Io is in darkness. This may indicate that some of the current flow between Io and Jupiter is conducted through the interior of Io, particularly during periods when the atmospheric conductivity is low.

The Galileo spacecraft has been orbiting Jupiter and its moons since December 1995. Galileo is managed by the Jet Propulsion Laboratory, Pasadena, Calif., for NASA.